Water Methanol

In internal combustion engines, Water Methanol injection, also known as anti-detonant injection (ADI), can spray water into the incoming air or fuel-air mixture, or directly into the cylinder to cool certain parts of the induction system where “hot points” could produce premature ignition.

Water injection was used historically to increase the power output of military aviation engines for short durations. However it has also been used in motor sport and notably in drag racing. In Otto cycle engines, the cooling effects of water injection also enables greater compression ratios by reducing engine knocking (detonation). Alternately, this reduction in engine knocking in Otto cycle engines means that some applications gain significant performance when water injection is used in conjunction with a supercharger, turbocharger, or modifications such as aggressive ignition timing.

Theory

Water has a very high heat of vaporization. As the ambient temperature water is injected into the engine, heat transfers from the hot cylinder head/intake air into the water. This makes it evaporate, cooling the intake charge. A cooler intake charge means it is more dense (higher volumetric efficiency) and also has a lower tendency to knock. However, the water vapor displaces some air, negating some of the denser intake charge benefit. Knocking is generally more of a problem in forced induction engines rather than naturally aspirated, so this can help prevent it. On electronic ignition systems the ignition timing is generally retarded to prevent knock from occurring but with water injection it can be advanced closer to Maximum Brake Torque (MBT) timing for additional power.